This invention relates to apparatus for the purification of blood, for example, by extracorporeal hemodialysis and/or hemofiltration and/or hemoperfusion. Hemodialysis is a widely used medical treatment for patients with acute and chronic renal failure. Hemofiltration is an alternative treatment gaining in popularity which involves massive removal of plasma water (approximately 30 liters) and replacement with sterile electrolyte solution. Hemoperfusion is a process for detoxifying blood of, for example, drug overdose, by contacting the blood with an adsorbent, such as activated carbon, to remove the toxins.
Hemodialysis services are normally supplied on an out-patient basis in specially constructed renal dialysis units or carried out at home by the patients themselves. Patients typically require three sessions of 4-6 hour duration per week. The hardware associated with hemodialysis tends to be provided as a fixed installation with the necessary plumbing and control apparatus.
Belgian Patent Number 905,407 (filed, 10th Sept. 1986, published 10th Mar. 1987) describes a single-needle hemodialysis system in which blood withdrawn by a pump from a patient suffering from renal insufficiency is passed through a dialyzer into an expansion chamber. The expansion chamber is in the form of a solid-walled, pressure resistant vessel having a line through which blood is delivered to and withdrawn from the vessel and also on air line extending from the vessel to a pressure sensor which has an associated control system operatively connected to the pump. As blood is delivered into the chamber by the pump the pressure of air, trapped in the vessel above the blood level, increases and is sensed by the sensor. When a preselected pressure is sensed by the pressure sensor, the control system sends a signal to the pumpp to reverse the direction of pumping thereby withdrawing the blood from the expansion chamber and returning it, via the dialyzer and pump, to the patient. Thereafter, when the sensed pessure falls to a preselected lower limit the control again reverses the direction of pumping to complete the cycle. A disadvantage of the proposed system is the danger inherent in the failure of the control which may result in dangerous, even fatal, amounts of air being introduced into the patient's circulation.
The most widely used hemodialysis system involves withdrawal of blood from the patient via a needle introduced into a vein supplied by a surgically created arteriovenous fistula, establishing a flow, using a peristaltic pump through a dialyzer with return to the patient's circulation via a second needle. Alternatively, a single needle is used in which case blood being taken from the fistula and returned to the fistula alternately by using twin pumps. A flow of dialysis fluid is established through the dialyzer across the remote side of the membrane from the blood and thence to drain. In order to achieve sufficient dialysis within an acceptable time frame, flow rates of both blood and dialysis fluid are high, typically from 200 ml/min to 400 ml/min for blood and 500 ml/min for dialysis fluid. The extra-corporeal blood volume of the dialysis system is high (300-450 ml) and large volumes of the dialysis fluid are required (120-160 liters per dialysis). The overall size of the installation and the complexity of the instrumentation is consequently great. It is these requirements along with the need for large volume of dialysis fluid which operate against portable units.
The foregoing description applies particularly to treatment of chronic renal failure. However, in acute renal failure which is a reversible condition which sometimes follows trauma, surgery or severe illness, patients may require blood purification during their stay in intensive care units. In this setting, a bulky dialysis machine and its associated parts and the need for special plumbing is, as best, inconvenient because of competition for space with other intensive care equipment such as respirators, drips and vital signs monitors. Also the complexities of operation of traditional dialysis machines cause problems for intensive care nursing staff who may not have been trained in renal units.